1. Field of the Invention
The present invention relates to a dual spin valve sensor without antiferromagnetic (AFM) pinning and, more particularly, to such a sensor which has first and second antiparallel (AP) pinned layer structures on each side of a free layer structure with thicknesses that permit a sense current field to pin the magnetizations of the layers of the AP pinned layer structures so that the layers next to the free layer structure are pinned parallel with respect to one another.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry, which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor are a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk.
The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced. Readback asymmetry is defined as             V      1        -          V      2            max    ⁡          (                        V          1                ⁢                  xe2x80x83                ⁢        or        ⁢                  xe2x80x83                ⁢                  V          2                    )      
For example, +10% readback asymmetry means that the positive readback signal V1 is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in some applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry.
The location of the transfer curve relative to the bias point is influenced by four major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demagnetizing (demag) field HD from the pinned layer, a sense current field HI from all conductive layers of the spin valve except the free layer, a net image current field HIM from the first and second shield layers. The strongest magnetic force on the free layer structure in a single spin valve sensor is the sense current field HI. In an exemplary bottom spin valve sensor where the free layer is closer to the second gap layer than it is to the first gap layer the majority of the conductive layers are below the free layer structure between the free layer structure and the first gap layer. The amount of conductive material in this region is further increased if the pinning layer is metal, such as platinum manganese (PtMn), instead of an oxide, such as nickel oxide (NiO). When the sense current is conducted through the sensor the conductive layers below the free layer structure cause a sense current field on the free layer structure which is minimally counterbalanced by a typical cap layer made of tantalum (Ta) on top of the free layer structure. Further, the pinned layer structure below the free layer structure in a bottom spin valve sensor exerts a demagnetizing field on the free layer structure which needs to be counterbalanced to improve asymmetry of the spin valve sensor.
A dual spin valve sensor may be employed for increasing the magnetoresistive coefficient dr/R of a read head. In a dual spin valve sensor first and second pinned layers are employed with a first spacer layer between the first pinned layer and the free layer and a second spacer layer located between the second pinned and the free layer. With this arrangement the spin valve effect is additive on each side of the free layer to increase the magnetoresistive coefficient dr/R of the read head. A distinct advantage of the dual spin valve sensor is that the free layer structure is in the middle of the sensor so that sense current fields from the pinned layer structures and other conductive layers on each side of the free layer structure counterbalance each other. Accordingly, the net sense current field acting on the free layer structure can be made to be zero or nearly zero. A disadvantage of the dual spin valve sensor, as compared to the single spin valve sensor, is that it is significantly thicker than the single spin valve sensor which means that it has a larger read gap as measured between the first and second shield layers. The larger read gap equates to a reduced read bit density of the read head. In a dual spin valve sensor a first pinning layer pins the magnetic moment of the first pinned layer structure and a second pinning layer pins the magnetic moment of the second pinned layer structure. Each of the pinned layers is required to have at least a thickness of 125 xc3x85 in order to implement the required pinning of the pinned layer structures. Accordingly, each pinning layer has a thickness which is greater than the combined thicknesses of all of the layers of the dual spin valve sensor. This means that less data can be stored on a rotating magnetic disk which results in less storage capacity of the computer. Accordingly, there is a strong-felt need to obtain the increased magnetoresistive coefficient dr/R of a dual spin valve sensor without sacrificing linear bit density of the read head.
The present invention provides a dual spin valve sensor with a significantly decreased sensor stack as compared to prior art dual spin valve sensors. The present dual spin valve sensor omits the prior art first and second pinning layers so as to reduce the overall sensor stack height by at least 250 xc3x85. This is accomplished by providing the dual spin valve sensor with uniquely configured first and second antiparallel (AP) pinned layer structures wherein a first AP pinned layer structure is located between the first gap layer and the free layer structure and the second AP pinned layer structure is located between the second gap layer and the free layer structure. For sake of clarity, the first AP pinned layer structure will be described as having an antiparallel (AP) coupling layer located between ferromagnetic first and second AP pinned layers and the second AP pinned layer structure will be described as having an AP coupling layer located between ferromagnetic third and fourth AP coupling layers. During fabrication of such a sensor the first AP coupling layer will be formed after formation of the first gap layer followed by the second, third and fourth AP coupling layers. In the present dual spin valve sensor the magnetic moments of the first, second, third and fourth AP pinned layers are pinned by a sense current field from the sense current IS. It is important that this pinning action pin the magnetic moments of the second and third AP pinned layers parallel with respect to one another so that they are in-phase with respect to one another. When they are in-phase the spin valve effects on each side of the free layer structure will be additive. This is accomplished by making the first and third AP pinned layer structures thinner or thicker than the second and fourth AP pinned layer structures respectively. When the thicknesses of the AP pinned layers are fashioned according to the present invention, the dual spin valve sensor is insensitive to the direction of the sense current. Accordingly, for either direction of the sense field through the spin valve sensor the second and third AP pinned layers next to the free layer structure will be in-phase. However, after selecting a particular direction for the sense current an aspect of the present invention is to fabricate the AP pinned layers with their magnetizations oriented in the same direction as the fields which pin their directions when the sense current IS is conducted through the dual spin valve sensor. Accordingly, during fabrication of the AP pinned layers, which is typically done by sputtering, a magnetic field is directed in the pinned direction of each AP pinned layer during the sputtering of the layer.
Since the sense current field of the spin valve sensor is substantially balanced, as discussed hereinabove, this leaves a ferromagnetic coupling field HFC which must be balanced in order to provide a zero bias point for the free layer structure. In a dual spin valve sensor there is a ferromagnetic coupling field on each side of the free layer structure due to the AP pinned layer which is next to each side of the free layer structure. Each of the ferromagnetic coupling fields is typically positive which means its direction is parallel to the direction of the magnetization of the respective AP pinned layer. This means that the ferromagnetic coupling fields are additive into the sensor or out of the sensor, depending upon the directions of the magnetizations of the respective AP pinned layers. The net ferromagnetic coupling field, which is a summation of the two ferromagnetic coupling fields on each side of the free layer structure, is counterbalanced in the present invention by a net demagnetization field from the first and second AP pinned layer structures. This is accomplished by making the net magnetic thickness of the first AP pinned layer structure either greater or less than the magnetic thickness of the second AP pinned layer structure.
Another distinct advantage of the present dual spin valve sensor is that the magnitude of the sense current IS is not restricted by the blocking temperatures of the first and second pinning layers employed in the prior art dual spin valve sensor. If the sense current IS was too high in the prior art dual spin valve sensor the temperature of the sensor can be raised to or near to the blocking temperature of either of the pinning layers which causes the pinning layer to lose its exchange coupling with the adjacent pinned layer structure. When the pinned layer structure loses its pinning from the pinning layer the magnetic moment of the pinned layer structure is free to move in various directions which renders the read head unstable and incapable of performing the reading function. With the present dual spin valve sensor the sense current IS can be elevated which equates to an increase in the output signal of the dual spin valve sensor. The stripe height of the sensor, which is the distance between the ABS and the recessed end of the sensor in the head, is also not restricted by the blocking temperatures. While a decrease in the stripe height increases heat within the spin valve sensor, it also increases the sense current field HI which is desirable for implementing a strong pinning action of the AP pinned layer structures. Another distinct advantage of the present dual spin valve sensor is that the absence of the first and second pinning layers, which are typically conductive, increases the sheet resistance of the sensor stack. Accordingly, the sense current is not shunted through the pinning layers which results in a higher magnetoresistive coefficient dr/R.
An object of the present invention is to provide a dual spin valve sensor which has an improved linear read bit density.
Another object is to provide the aforementioned spin valve sensor with net ferromagnetic coupling and demagnetization fields which counterbalance each other.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.